ISE’s Ion Selective Electrodes

Why would I want to measure specific ions?

Ion-Selective Electrodes (ISE’s) are a class of sensors that respond to the presence of specific ions, such as sodium (Na+) or bromide (Br). They are used in conjunction with a reference electrode, and report ion concentrations in mg/L.

The most common ISE’s measure ammonium (NH4+), nitrate (NO3-), or chloride (Cl-). Ammonium and nitrate are important because they are indicators of nitrogen-based nutrients. They are often used to detect contamination by wastewater treatment plants or agricultural activities. Chloride measurements are often used as indicators of contamination by sea water or other contamination sources containing chloride levels higher than expected in natural waters. There are roughly 30 other ISE’s available; some work better than others.

Changes in long-term, specific-ion trends can signal the need for more detailed chemical study of the water and its contamination sources.

How are specific ions measured?

There are two types of ISE’s. The “solid-state” ISE’s are constructed of a solid billet of material. An example is the chloride sensor, which is little more than a small piece of silver. Different chloride ion concentrations produce different voltage potentials at the surface of the silver, and these voltages are measured using the same reference electrode as a pH sensor.

The “liquid membrane” ISE’s are constructed of a thin, polymer membrane covering an electrolyte that contains an electrode wire. The membrane is “doped” with various materials that respond to the presence of specific ions, again by creating a voltage potential related to the concentration of the ion. And as with the solid-state ISE’s, these voltages are measured using the same reference electrode as a pH sensor.

Unfortunately, many specific-ion sensors are not particularly specific. Ammonium ISE’s, for instance, suffer interference from ions having similar size and charge (+1). Potassium is a minor (usually) interferent in fresh water, but in sea water, sodium ions often cause a false reading up to 12 mg/l-N ammonium even if there is no ammonium present.

For this reason, ammonium and nitrate sensors do not work in sea water.

Some ionic species are influenced by the pH of the sample water. Ammonium, for instance, converts into ammonia gas (NH3) at high pH (the pKa for ammoniumammonia is 9.3). This phenomenon is not related to ISE operation, but by measuring ammonium with an ISE and measuring pH, you can calculate the ammonia concentration, and total ammonia (ammonium plus ammonia) in units of mg/L-N (that’s mg/l of nitrogen whether in the form of ammonium or ammonia). This is convenient when operating at high pH where there is likely to be both ammonia and ammonium species present.

ISE’s are easy to calibrate unless you want to do it right. That involves a fourpoint calibration for higher accuracy – two concentrations at two temperatures. Most people choose instead to calibrate with two concentrations at a temperature near that expected for the actual measurements. Follow the instrument’s instructions. Calibration standards are available for many different concentrations.

Most ISE’s screw into a receiver with a simple o-ring seal. This makes it easy to change the sensor tips when their lifetime has been exhausted. Rev – 9.20 Please see below, “The Difference between Activity and Concentration when Calibrating Ion-Selective Electrodes” to understand how calibration standards are used.

What should I know about ISE measurement in the field?

ISE’s tend to drift far out of accuracy specification before they can accumulate fouling. Each user must assess the performance of an ISE in particular situations to determine how long an ISE can operate before exceeding a desired accuracy range. It’s best not to let your ISE dry out, so place a small amount of tap water in the sensor storage cup to ensure 100% humidity.